Synthetic polymer compounds have been widely used as plastics due to their superior characteristics. With the increase in the consumption of the synthetic polymer compounds, however, the amount of waste has been also increasing. It has thus become a social problem how the waste plastics should be dealt with. Incineration causes problems, such as the damage to furnaces due to high heat generation and the risk of emission of toxic substances. Landfill also cause a problem that the plastics stay in the environment for good since they do not decay. Furthermore, considering the costs of classification, collection and regeneration, it is difficult to completely solve the problem only by recycling.
In the midst of such increasing environmental concern, needs for biodegradable plastics which decompose in the natural environment after being wasted have been increasing to reduce environmental load and to realize the sustainable society.
Known biodegradable plastics include starch-based ones, aliphatic-polyester resins produced by microorganisms, chemically synthetic aliphatic polyester resins and the same resins but partially modified in their chemical structure, and biodegradable aliphatic aromatic polyester resins.
Among these biodegradable plastics, polylactic acid resins have superior transparency, rigidity and processability, as compared with other biodegradable plastics. In particular, oriented films of polylactic acid resins are suitable as various films for packaging such as bags, window films for containers with window, plastering films for outlook window envelopes, substitute films for cellophane, and the like, due to their high stiffness and high transparency.
On the other hand, in the field of plastic film applications, processing speed has been increasing with the progress of processing machinery. For example, in the affixing of films for envelope windows, recent processing speed is 800 to 1,000 sheets/minute, or even 1000 or more sheets/minute due to the progress of window affixing machine, while the conventional processing speed was 400 to 600 sheets/minute. For this reason, running speed of the film also has been increasing and thus there are needs for a film with superior high-speed cutting property. In addition, there are applications in which film surface is to be coated with an antistatic agent, a lubricant, an antiblocking agent, or the like, to provide the film with machine adaptability and processability to suit the high-speed processing. For example, the above-described coating is indispensable in bags, packaging films and films for various windows, particularly those for window of outlook window envelopes into which relatively light weight materials easily affected by static electricity, such as powder, granule, thin papers, film, fiber-like substance, are contained. Therefore, the films are required to have coating adaptability, in addition to the high-speed cutting property. Biodegradable films, in particular polylactic acid film, with superior transparency and mechanical properties, satisfying these requirements have not been obtained.
JP-A-2001-122989 discloses that a biaxially oriented polylactic acid polymer film consisting of crystalline polylactic acid and having the storage modulus E′ at 120° C. within the range from 100 MPa to 230 MPa, as determined by the test method for temperature dependency of dynamic viscoelasticity, is suitable for fold-packaging due to superior tacking property. This film, however, cannot satisfy the coating adaptability and high-speed cutting property. JP-A-2000-198913 discloses an easily tearable, biaxially oriented film of a polylactic acid resin consisting of polylactic acid and crystalline aliphatic polyester. This film, however, only has limited applications due to significant haze. JP-A-2001-64413 discloses an easily tearable, biaxially oriented film of a polylactic acid resin consisting of polylactic acid and polyethylene terephthalate and/or polyethylene isophthalate. This film, however, only has limited applications due to incomplete biodegradability and insufficient coating adaptability and high-speed cutting property, although it is superior in straight tearing and hand tearing properties. JP-A-2001-354789 also discloses biodegradable polylactic acid resin films with a good balance among antistatic property, lubricity and antiblocking property, along with superior adhesion to paper. This film, however, does not have the high-speed cutting property.
The object of the present invention is to provide a biodegradable, biaxially oriented film of a polylactic acid resin suitable for films for bags, packaging and films for various windows with superior coating adaptability for antistatic agents, lubricants, antiblocking agents or the like, as well as high-speed cutting property.